Chemically bound emulsifier

The microemulsion polymerization and copolymerization of amphiphilic monomers and macromonomers can produce the fine polymer latex in the absence of emulsifier [98-100], The surface active block or graft copolymer stabilizes the latex particles. The chemically bound emulsifier (surface active copolymer) onto the particles surface is known to be much more efficient emulsifier than the emulsifier physically adsorbed onto the particle surface and, therefore, very stable and fine polymer latexes are formed. The similar behavior is expected with the transferred emulsifier radicals. For example, the surface-functionalized nanoparticles in the 12 - 20 nm diameter range can be prepared by a one-step or two-step microemulsion copolymerisation process of styrene (and/or divinylbenzene (DVB)) with the polymerisable macromonomer (Fig. 7) [93, 101]. 

The second generation includes latices made with functional monomers like methacrylic acid, 2-hydroxyethyl acrylate [818-61 -17, acrylamide/75 -(9ti-/7, 2-dimethylaminoethylmethacrylate [2867-47-2] and sodiumT -vinyl-benzenesulfonate [98-70-4] that create in polymeric emulsifier. The initiator decomposition products, like the sulfate groups arising from persulfate decomposition, can also act as chemically bound surfactants. These surfactants are difficult to remove from the latex particle. 

The emulsifier provides sites for the particle nucleation and stabilizes growing or the final polymer particles. Even though conventional emulsifiers (anionic, cationic, and nonionic) are commonly used in emulsion polymerization, other non-conventional ones are also used they include reactive emulsifiers and amphiphilic macromonomers. Reactive emulsifiers and macromonomers, which are surface active emulsifiers with an unsaturated group, are chemically bound to the surface of polymer particles. This strongly reduces the critical amount of emulsifier needed for stabilization of polymer particles, desorption of emulsifier from particles, formation of distinct emulsifier domains during film formation, and water sensitivity of the latex film. 

In order to synthesize an interfacial polyelectrolyte the ionizable groups must be chemically bound to the interface and be an integral part of the polymer molecules comprising the colloidal particles. It is preferable to avoid adsorbed emulsifier, which is usually employed to stabilize these colloids, as it complicates the subsequent purification and quantitative surface characterization of the system. Thus the classical technique of emulsion polymerization, as described by Harkins [2], is not applicable. The systems are actually simpler, employing only monomer(s), water and initiator. It is most important that the initiator be very soluble in the water and that it form ionizable free radicals. The monomer must have a finite solubility in water, although this may be very small (even styrene satisfies this requirement at 0.038 wt.%), but may be present in amounts far in excess of this as an initially separate phase. Initiation of polymerization must then occur in the aqueous solution. For instance, if the initiator is potassium persulfate, K2S2O8, free radical-ions are formed (along with some OH) by the thermal decomposition of the anion ... 

Therefore, we decided to remove the adsorbed emulsifier as completely as possible and to rely upon the sulfate endgroups to give the particle the required stability. This would give an ideal model colloid, i.e., monodisperse spheres of constant and known surface charge arising from chemically-bound strong-acid surface groups. 

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